Automatic test equipment for checking printed circuit boards has long involved use of a "bed of nails" test fixture to which the circuit board is mounting during testing. This test fixture includes a large number of spring-loaded test probes arranged to make electrical contact under spring pressure with designated test points on the board under test. Any particular circuit laid out on a printed circuit board is likely to be different from other circuits, and consequently, the bed of nails arrangement for contacting test points in a particular board must be customized for that circuit board. When the circuit to be tested is designed, a pattern of test points to be used in checking the board is selected, and a corresponding array of test probes is configured in the test fixture. This typically involves precision-drilling a pattern of holes in a probe plate to match the customized array of test probes, and then mounting the test probes in the drilled holes on the probe plate. The circuit board is then mounted on the fixture, superimposed over the array of test probes. During testing, the spring loaded test probes are brought into spring pressure contact with the test points on the board under test. Electrical test signals are transferred from the board to the test probes and then to the exterior of the fixture for communication with a high speed electronic test analyzer which detects continuity or lack of continuity between the various test points in the circuits on the board.
Various approaches have been used in the past for bringing the test probes in the circuit board into pressure contact for in-circuit testing. One class of these fixtures is a wired test fixture in which the test probes are individually wired to separate interface contacts for use in transmitting test signals from the probes to the external test analyzer. These wired test fixtures are often referred to as "vacuum test fixtures" since a vacuum is applied to the interior of the test fixture housing during testing to draw the circuit into contact with the test probes. Pneumatic or mechanical type test fixtures are also popular. In a vacuum fixture, a moveable top plate is mounted over the stationary probe plate and a vacuum seal is formed between the top plate and the probe plate. A second vacuum seal is mounted above the top plate and has a sufficient height to hold the printed circuit board above the spring probes which project through access holes drilled in the top plate for alignment with the underside of the board. During use, a vacuum applied to the region between the probe plate and the top plate is also applied the underside of the board. This compresses both vacuum seals and pulls the board down against and into electrical contact with the test probes. By maintaining the vacuum seal, the probes are held in spring-pressure contact with the test points on the board while the board is tested.
In order for the probes to make contact with the proper test points of the circuit board, the bottom stationary probe plate and the moveable top plate which supports the board must remain in a parallel relationship, to hold the board flat while maintaining its position perpendicular to the probe field. A reliable vacuum seal also is necessary.
A further class of test fixtures is also called "dedicated" test fixtures, also known as "grid-type fixtures" in which the random pattern of test points on the board are contacted by translator pins which transfer test signals to interface pins arranged in a grid pattern in a receiver. In the grid-type testers, fixturing is generally less complex and simpler than in the customized wired test fixtures; but with a grid system, the grid interfaces and the test electronics are substantially more complex and costly. The interface pins are generally single-ended spring probes which are retained within the receiver.
The spring probes of the vacuum fixtures and the grid-type fixtures have been retained within the fixtures by two previous methods. In a first method the receptacle of the spring probe includes a press ring and is rigidly mounted within a hole in the probe plate. The press rings retains the receptacle and consequently the spring probe within the fixture. Detents in the receptacle allow the probe plunger to be inserted and retained within the receptacle. Electrical connection to the probe assembly is most commonly made using a wire wrap around the square pin installed in the receptacle. Other common electrical connections are made using crimped wire, push on terminals, or by soldering a wire to the receptacle. Double-ended spring probes are also used in on-grid testing wherein the receptacle of the double-ended spring probe is firmly secured to the probe plate by a press ring. Electrical connection to the double-ended spring probe assembly is most commonly made by using a printed circuit board installed internally, to replace the wires. These are commonly referred to as wireless fixtures. The probe on one end contacts the unit under test, while the probe on the other end, makes electrical contact with the fixture printed circuit board.
A second method of retaining the test probes is by the use of a Mylar sheet wherein the spring probes extend through and below the receiver or probe plate through holes in the receiver or probe plate and are maintained by the Mylar sheet which is rigidly held below the receiver or probe plate. The spring probes include a wide groove, which is a reduced diameter roll on the probe barrel and the Mylar sheet fits in the groove which allows the probe to move up and down with respect to the receiver or probe plate equal to the height of the groove.
Both of these types of spring loaded test probe retention systems includes disadvantages and drawbacks due to current trends in the printed circuit board testing environment. One problem encountered is current probe/receptacle assemblies are prone to inaccuracies. The receptacle may be installed at an angle within the hole through the receiver or probe plate, or may tilt or pivot about the press ring within the hole. Further inaccuracy can be created due to the height at which the press ring is positioned within the receiver or probe plate. In addition, the probe is positioned within a barrel within the receptacle creates inaccuracies due to variations in materials and the need for clearance for the probe plunger to slide within the barrel and the barrel to slide within the receptacle. Another problem encountered is that by having two tubes mounted concentrically also reduces the available area for the spring which reduces possible higher spring force and spring life for the probes. Furthermore the use of spring probes having a receptacle with press rings is contrary to the increasing trend of miniaturization. The projecting press rings on the receptacle reduces the capability of close spacing of the spring probes which is undesirable if the required pin density must be increased to match a tight density spacing of test points.
Another problem is that a double-ended spring probes are prone to damage, due to the small size of the bottom probe and the assembly process. In most instances, the plate that holds the double-ended probes and the fixture printed circuit board are assembled using stand-offs. If the screw that pulls the printed circuit board into place through the stand-offs are not tightened evenly, the probes will be damaged or broken. This can be a serious problem in that wireless fixtures need to be disassembled to replace broken or damaged spring probes.
The current trend of miniaturization has resulted in a trend to use "no clean flux" for electrical connections. The use of this type of flux tends to have more contamination covering the test targets, which makes achieving reliable electrical contact more difficult. The industry solution is to use higher spring force and/or sharper probes. A suitable high spring force is unfortunately difficult to achieve with conventional small diameter spring probes.
Yet another problem encountered with conventional test fixtures due to the current methods of retaining spring probes is high cost. The additional manufacturing steps associated with roll forming a groove in the barrel for use with Mylar or manufacturing press rings is multiplied when you consider that thousands of test probes can be used within a particular test fixture. In addition double-ended probes are by nature more expensive. Consequently, a need exists for an improved method of retaining test probes within a test fixture which addresses the problems created by prior art methods.